[Blacdout96]

Heres the info I got form a site that sells rebuilt heads:

The returned cylinder heads MUST be from a late 1998 or newer engine as prior to mid-1998, GM used smaller valvespring pilot seats and the heads were prone to cracking at these locations. The core heads must include valves and be in rebuildable condition. Seals, springs retainers or keepers need not be included as we replace these.